The color comes from Thiocyanatoiron ions. Skin was rubbed with a cotton ball soaked with a solution of thiocyanate ions, and that knife was covered with a solution [form] of iron(III), which has yellowish color. When solutions have combined, these ions reacted together forming blood red [Fe(SCN)]2+.

And in case you were wondering, you're too late: Someone already made the "class cuts you" joke.